1. The Field of the Invention
The present invention relates to x-ray generating devices. More particularly, the present invention relates to an x-ray tube having an integral housing assembly that allows for improved performance, reliability, safety and patient comfort.
2. The Relevant Technology
X-ray devices are extremely valuable tools for use in a variety of medical applications. For example, such equipment is commonly used in areas such as diagnostic and therapeutic radiology.
Regardless of the particular application involved, the basic operation of medical x-ray devices is similar. In general, x-rays, or x-ray radiation, are produced when electrons are produced, accelerated to high speeds, and then stopped abruptly. Typically, this entire A process takes place within an x-ray tube housing that defines an evacuated envelope. This evacuated envelope is typically constructed of glass, metal, or a combination of metal and glass. Disposed within the evacuated envelope is a cathode assembly, which produces the electrons, and an anode assembly, which is axially spaced apart from the cathode and oriented so as to receive electrons emitted by the cathode.
In operation, a voltage potential is applied between the cathode and the anode. This potential causes the electrons that are emitted from the cathode filament to form a thin stream or beam, and accelerate to a very high velocity towards a target surface positioned on the anode. This target surface (sometimes referred to as the focal track) is comprised of a refractory metal having a high atomic number, so that when the electrons strike it, at least a portion of the resulting kinetic energy is converted to electromagnetic waves of very high frequency, i.e., x-rays. The resulting x-rays emanate from the target surface, and are then collimated for penetration into an object, such as an area of a patient""s body. As is well known, the x-rays that pass through the object can be detected and analyzed so as to be used in any one of a number of applications, such as a medical diagnostic examination.
In general, a very small part of the input energy results in the production of x-rays. A majority of the kinetic energy resulting from the electron collisions at the target surface is converted into heat, which can reach extremely high temperatures. The heat is absorbed by the anode and is conducted not only to other portions of the anode assembly, but to the other x-ray tube components within the evacuated envelope. Over time, this heat can damage the anode, the anode assembly, and/or other tube components, and can reduce the operating life of the x-ray tube and/or the performance and operating efficiency of the tube.
Several approaches have been used to help alleviate problems arising from the presence of these high operating temperatures. For example, in some x-ray devices the x-ray target, or focal track, is positioned on an annular portion of a rotatable anode disk. The anode disk (also referred to as the rotary target or the rotary anode) is then mounted on a supporting shaft and rotor assembly that can then be rotated by a motor. During operation of the x-ray tube, the anode disk is rotated at high speeds, which causes the focal track to continuously rotate into and out of the path of the electron beam. In this way, the electron beam is in contact with any given point along the focal track for only short periods of time. This allows the remaining portion of the track to cool during the time that it takes to rotate back into the path of the electron beam, thereby reducing the amount of heat absorbed by the anode.
While rotation of the anode reduces the amount of heat present at the focal spot on the focal track, a large amount of heat is still transferred to the anode, the anode drive assembly, and other components within the evacuated housing. This heat must be continuously removed to prevent damage to the tube (and any other adjacent electrical components) and to increase the x-ray tube""s efficiency and overall service life.
One approach has been to place the housing that forms the evacuated envelope within a second outer metal housing, which is sometimes referred to as a xe2x80x9ccan.xe2x80x9d This outer housing or can serves several functions. First, it acts as a radiation shield to prevent radiation leakage. As such, it must be at least partially constructed from some type of dense, x-ray absorbing metal, such as lead. Second, the outer housing serves as a container for a cooling medium, such as a dielectric oil, which is circulated by a pump over the outer surface of the inner evacuated housing. As heat is emitted from the x-ray tube components (anode, anode drive assembly, etc.), it is radiated to the outer surface of the evacuated housing, and then at least partially absorbed by the coolant fluid. The heated coolant fluid is then passed to some form of heat exchange device, such as a radiative surface, and the heat is removed. The fluid is then re-circulated by the pump back through the outer housing and the process repeated.
The dielectric oil (or similar fluid) is also often relied upon to provide functions other than cooling. For example, the oil serves as an electrical insulator between the inner evacuated housing, which contains the cathode and anode assembly, and the outer housing, which is typically comprised of a conductive metal material. The presence of the fluid insulator reduces the possibility of electrical arcing between the evacuated housing and the outer housing, and also provides electrical insulation between any high voltage leads connected to the evacuated envelope.
While useful as a heat removal medium and/or as an electrical insulator, the use of oil and similar liquids can be problematic in several respects. For example, use of a fluid adds complexity to the construction and operation of the x-ray generating device in several areas. First, use of fluid requires that there be a second outer housing or can structure to retain the fluid. This outer housing is constructed of a material that is capable of blocking x-rays, and it must be large enough to be completely disposed about the inner evacuated housing and allow fluid to be disposed therein. This increases the cost and manufacturing complexity of the overall device. Also, the outer housing requires a large amount of physical space, resulting in the need for an overall larger x-ray generating device. This can limit the device""s ability to be used in close proximity to a patient and/or can increase discomfort to the patient during certain types of procedures.
Also, the space required for the outer housing reduces the amount of space that can be utilized by the inner evacuated housing, which in turn limits the amount of space that can be used by other components within the x-ray tube. For example, the size of the rotating anode is limited; a larger diameter anode is often desirable because it is better able to dissipate heat as it rotates.
The need for an outer housing adds expense and manufacturing complexity to the overall device in other respects. When liquid is used as a coolant, the device may need a pump and a radiator (or similar heat removal device), that in turn must be interconnected within a closed circulation system via a system of tubes and fluid conduits. Also, since the oil expands when it is heated, the closed system must provide a facility to expand, such as a diaphragm or similar structure. Again, these additional components add complexity and expense to the x-ray device""s construction. Moreover, the tube is more subject to fluid leakage and related catastrophic failures attributable to such a fluid system.
The presence of a liquid coolant/dielectric is also detrimental because it does not function as an efficient noise insulator. In fact, the presence of a liquid may tend to increase the mechanical vibration and resultant noise that is emitted by the operating x-ray tube. This noise can be distressing to the patient and/or the operator. The presence of liquid also limits the ability to utilize other, more efficient materials for dampening the noises emitted by the x-ray tube due to space restrictions and the need for effective electrical insulation.
Use of a liquid coolant gives rise to safety concerns as well. In particular, during operation, the temperature of the coolant reaches extremely high temperatures. The structures containing the fluid must therefore be extremely robust to insure that there is never any accidental leakage. This need is especially acute since the x-ray tube is often in very close proximity to a patient. Obviously, any leakage could be catastrophic.
Use of liquid coolant is problematic in yet another respect. In particular, the need to dispose of a dielectric oil, as well as the lead-lined outer housing, gives rise to a number of environmental concerns. In particular, the disposal of such materials is often governed by strict local and national regulations. Compliance is often expensive and time consuming, which adds to the cost of using such equipment.
Some prior art x-ray tubes have eliminated the use of an outer housing and fluid as a coolant/dielectric medium. For example, some solutions utilize forced air to remove heat from the evacuated housing and its components. However, these approaches have not been entirely satisfactory for a variety of reasons. Also, proposed solutions are not well suited for certain types of x-ray applications, such as x-ray mammography and similar applications.
For example, known x-ray generating devices that utilize forced air as a cooling medium are adapted for high voltage x-ray applications; such applications typically utilize a 150 kV operating potential, or higher, between the anode and cathode. High operating voltages result in higher operating temperatures, and to ensure sufficient heat removal with air convection, these x-ray tubes typically are equipped with fins, or channels formed on the outer surface of the evacuated envelope so as to enhance heat removal. As with previous solutions, this need for additional structure increases manufacturing complexity, and involves additional physical space requirements for the assembly. Moreover, in these types of devices, since the outer housing is eliminated, the housing forming the evacuated enclosure must provide a sufficient level of radiation shielding. To do so at such higher operating voltage levels, the walls that form the enclosure must either be very thick, or must be constructed of more expensive materials. Again, this requires increased physical space and/or results in higher manufacturing costs.
In addition to the increased shielding capacity that must be provided by the walls of the evacuated enclosure, prior art devices must also provide additional shielding within the enclosure itself. For instance, openings are typically provided through the top and bottom portions of the evacuated housing, for example, to allow for the passage of electronic wires to the cathode assembly. Additional shielding structure must be provided so as to block any x-rays from escaping through these openings. Again, this adds to the amount of physical space that is available to other components, and increases manufacturing complexity of the x-ray tube.
Radiographic devices utilizing air cooling must also replace the dielectric oil as the means for electrically insulating the evacuated envelope (the cathode and the anode) from the rest of the assembly. Also, the device must provide some facility for reducing the amount of noise emitted by the x-ray tube during operation. As previously noted, the occurrence of noise resulting from a rotating anode can be especially troublesome to patients during some applications, such as mammography procedures.
Thus, what is needed in the art is a radiographic device that does not require the use of an outer housing for containing oils or similar fluids for the removal of heat and/or for providing an electrical insulator. Such a device would thereby eliminate the liabilities associated with the use of such fluids, such as increased manufacturing complexity, potential for failure, the need for increased physical space and problems associated with the proper disposal of the fluid. The device should also preferably maintain safe levels of radiation containment, and should also emit low amounts of audible noise during operation. Finally, the device must be extremely safe in all respects, and should present minimal environmental problems.
Briefly summarized, embodiments of the present invention are directed to an x-ray generating apparatus that eliminates the need for a liquid coolant contained within an outer x-ray tube housing or xe2x80x9ccan.xe2x80x9d Instead, embodiments utilize an x-ray tube having a single integral housing assembly that is capable of providing the vacuum enclosure that contains the cathode and anode assemblies. Moreover, the assembly is designed so as to provide a sufficient level of cooling and radiation blocking. In preferred embodiments, the x-ray generating apparatus of the present invention is particularly adapted for use in low power applications, where the energy potential between the anode and the cathode is approximately 25-30 kV, with an operating current at approximately 80-100 mA. These lower kV levels produce x-rays that have a lower energy spectrum, and the lower energy x-rays are better absorbed by softer breast tissue, resulting in an overall better contrast in the resulting x-ray image. However, it will be appreciated that embodiments of the invention can also be used with other applications and environments, including applications utilizing higher power. Also, embodiments of the present invention are applicable to a variety of voltage potential configurations, including a grounded anode mode, a grounded cathode mode, double ended mode, or any other appropriate combination depending on the needs of the particular application.
In one embodiment, the single integral housing is formed as a generally cylindrically shaped body. Supported on a cathode mounting structure within the interior of the housing is a cathode having an emission source, such as a filament, for emitting electrons. The cathode is supported so as to be positioned opposite from a focal track formed on a rotating anode. The focal track is positioned on the anode so that x-rays are emitted through a window formed through the side of the housing. In one embodiment, the cathode is freely supported on the cathode mounting structure, insofar as it is supported without the use of an oversized radiation shield or disk for blocking x-rays from exiting an opening formed through the housing. The elimination of a need for a larger x-ray blocking disk on the cathode frees up space within the interior of the housing for use by other components, and reduces overall manufacturing complexity and cost. In preferred embodiments, portions of the cathode itself are constructed with a radiation blocking material, which prevents x-rays from exiting the opening formed through the housing. In another preferred embodiment, the cathode mounting structure is shortened, allowing for the implementation of an x-ray tube having a shorter overall length. In another embodiment, the use of a cathode support arm is eliminated entirely, and the cathode is attached directly to the interior surface of the evacuated housing, or is integrated within the wall of the housing itself. Again, this shortens the length of the x-ray tube. In certain applicationsxe2x80x94especially mammographyxe2x80x94this can improve patient comfort, as well as allow for a greater freedom of maneuverability of the x-ray generating apparatus.
In one embodiment, at least a portion of the integral housing is formed of low cost material such as copper, stainless steel, alloys thereof, or any other material that possesses thermal conduction characteristics that allow heat to be absorbed from the anode assembly during operation, and then conducted to the outer surface of the integral housing. Also, at least that portion of the housing that is adjacent to the rotating anode includes walls that are of sufficient thickness so as to block x-rays, preferably in a manner so as to comply with applicable FDA requirements. When used in a lower power mammography application, the x-rays are of relatively lower intensity, and thus the wall thickness needed to shield x-rays is relatively lowxe2x80x94even with copper or stainless steel. Again, this reduces the overall size of the integral housing, as well as its cost. Other materials, including those that exhibit a higher degree of x-ray blocking characteristics, could also be used.
Preferred embodiments of the present invention utilize a forced air convection system to remove heat that is transferred to the outer surface of the integral housing, and to remove heat emitted from the stator, or motor assembly that is used to rotate the anode drive assembly. This eliminates the need for coolant fluids, such as dielectric oil and the like, and therefore eliminates the problems inherent with the use of such fluids. In one embodiment, a fan is used to direct air over the outer surfaces of the integral housing; preferably the airflow is directed with an airflow shell that is disposed about at least a portion of the integral housing. In some embodiments, a particular airflow to obtain optimal cooling can be provided by positioning air flow directors at specific locations within the airflow shell. The heat transfer characteristics of the integral housing, together with the cooling airflow, provide a sufficient level of heat removal so that the integral housing does not require external or internal fins, channels, or other similar means for conducting heat away from the surface. This, too, reduces manufacturing complexity, and reduces the overall physical size of the evacuated housing. It will be appreciated however that, depending on the needs of a particular application, such structures could be utilized to provide even further cooling of the integral housing.
Presently preferred embodiments of the present invention also include means for insulating the evacuated housing. In preferred embodiments, an electronic potting compound or encapsulant is used for the insulating means. For example, in one embodiment a dielectric gel is disposed between the integral housing and points external to the housing. A variety of other potting materials could be also used. Depending on the needs of the particular application, the material used preferably provides several functions. First, it the material may provide an electrical insulating function. For example, it may electrically insulate the high voltage (or groundxe2x80x94depending on the configuration) connection to the anode assembly, thereby preventing arcing and charge up of the evacuated integral housing (especially if there is a glass portion). Second, the material may preferably act as a dampening material that absorbs vibration and acoustical noise that originates from the anode rotor assembly. Reduced noise emissions are especially important to maintain the comfort of the patient and to help reduce any anxiety that would otherwise result from high noise emissions. Third, in some embodiments, the material may include an amount of a radiopaque material to provide additional levels of x-ray shielding to the integral housing. Finally, in preferred embodiments, the potting material is thermally conductive, so that it conducts heat away from surfaces it contacts. For example, the potting material would conduct heat away from portions of the evacuated housing, further enhancing the overall cooling of the operating x-ray tube.
In preferred embodiments, the insulating material, such as a potting compound, is positioned within selected areas of the x-ray tube assembly by way of an insulating shield positioned about portions of the evacuated housing. For example, the shield may be positioned about selected portions of the housing so as to define a gap between the shield and the outer surface of the housing. The potting material can then be placed within these gaps. The shield can also be used to form other chambered areas or volumes for containing the insulating material, depending on the needs of the application.
Some embodiments may optionally have additional features relating to the improved operation of the tube. For example, embodiments may include environmental sensors and controls for evaluating and controlling the operating environment within the evacuated housing, such as temperature regulation. Thus, in one embodiment, temperature sensors could be positioned within the evacuated housing, and airflow from a variable speed fan could be adjusted depending on temperature conditions. Alternatively, operation of the tube could be halted in the event that a maximum operating temperature is exceeded. Sensors for monitoring other environmental conditions, such as humidity could also be used. Also, devices that monitor electrical current and/or voltage levels could be utilized to monitor and protect electrical components, including the cathode filament, from various fault conditions such as filament over-current.
These and other advantages and features of the present invention will be apparent to those of skill in the art after having read the following detailed description of preferred embodiments, and the claims that follow.